Western Sydney airport link for commuters

The proposed Western Sydney Airport is not likely to have a rail link until at least a few years after it opens.

A study over the next year will determine the route, timing and funding options of a new passenger rail service for Sydney's west which will service commuters as well as Badgerys Creek airport users.

Acting Prime Minister Warren Truss said the link is unlikely to open at the same time as the airport, which will be serviced initially by an expanded road network.

"As the airport will start relatively small, it's not envisaged that there will be passenger rail transport in the early years," Mr Truss told reporters in western Sydney.

"You need to realise that the railway line is not a simple add-on to the airport - the railway line is going to cost more than the airport."

NSW Transport Minister Andrew Constance said he would prefer to have the train station open at the same time as the airport, but rigorous planning needed to be worked through.

"It would be tremendous to have it up and going from day one, but we need to make sure it's right, and that's what this government study will tease out," Mr Constance said.

With western Sydney's population tipped to rise from two million to three million in the next 20 years, Mr Truss said it was "absolutely essential" the region was serviced by passenger rail.

The joint federal and state government study will map out costs, time-frames and funding options for the rail link.